Disposal of hazardous waste is increasingly becoming a problem. The availability of suitable space for burial of such wastes is diminishing. Also, contamination of the environment by conventional methods of disposal, such as by dumping and incineration, is increasingly apparent. For example, metals, such as lead, which are often present in hazardous wastes, do not decompose. Release of hazardous wastes to the environment as gases and dust, such as fly ash, often contaminates water supplies and pollutes the atmosphere, thereby generally diminishing the quality of life in surrounding populations.
Those with heightened awareness of onsetting ecological problems are not only cognizant of the increasing problems of waste disposal but also of the impending hazards associated with the large volume of residual materials generated by the incineration of all forms of waste. Incineration generally results in incomplete combustion due to inefficient combination with oxygen and the presence of noncombustible inorganic salts and metals. This typically affords a residue in excess of twenty five percent of the initial mass of the waste material and often in excess of seventy percent. Since a mass balance must be maintained, except for the loss of mass due to water, nitrogen and related gases, the combination of carbonaceous materials with oxygen affording carbon dioxide actually increases the mass of off-gases by a factor of about three. Although the overall process achieves a desirable volume and mass reduction, the concentration of noncombustible toxic material is necessarily increased and is often contained within a leachable ash residue. Fly ash also continues to pose a more recognizable hazard since, as noted above, land-filling is being deemphasized as a viable disposal method. Attempts to reduce the volume of residue by more complete combustion is associated with an economic penalty due to post-combustion stages and the need for excess auxiliary fuels. Despite these efforts, the lower flame temperature indigenous to classical incineration methodologies combined with the associated temperature of their residuals are features that still assure that generic ash is typically below its fusion point and unvitrified. Such unvitrified residues are commonly leachable in nature and constitute an environmental hazard since the toxic constituents are now in a more concentrated form and may readily enter the ecosystem.
In addition, hazardous waste streams, such as fly ash produced by oil-fired thermal generating power stations, include valuable metals, such as nickel and vanadium. Attempts to recover such metals from fly ash have included, for example, smelting of fly ash in an arc flash reactor. However, chemical reduction of metal oxides in an arc flash reactor is generally incomplete. Further, other components of a metal-containing composition, such as hazardous waste-containing organic components, can release hazardous organic materials, such as dioxins, to the atmosphere during processing in an arc flash reactor.
Direct metal reduction technology involves directing a reducing agent, such as carbon, through the molten bath to thereby chemically reduce metal oxides. The overall objective is to selectively reduce oxides contained in ore as rapidly as possible to reduce production costs. The reducing agent, typically, is injected into, or more commonly onto, a molten bath at a rate and in an amount sufficient to react with metal oxides which are not dissolved in the molten bath. Reactions carried out under such heterogeneous conditions are highly inefficient because of a relatively low collision frequency of reactant molecules and cannot ensure a chemically remediated reduced species for recovery. Consequently, the amount of reducing agent which is introduced to the molten bath typically is significantly in excess of the theoretical amount required to chemically reduce the metal oxide. The rate at which the reducing agent is introduced to the molten bath is often sufficient to entrain metal oxides before they can dissolve in the molten bath for reduction therein. Such entrainment further diminishes chemical remediation of the remediable species.
Therefore, a need exists for a method that reduces and chemically remediates a metal oxide contaminant of a metal-containing composition in an ecologically sound manner which overcomes or minimizes the above-mentioned problems.